A team of researchers from Japan, Canada, the UK and the US has found that anthropogenic forcing—i.e., human-induced global warming—has contributed “significantly” to observed increases in precipitation in the Northern Hemisphere mid-latitudes, to drying in the Northern Hemisphere subtropics and tropics, and to moistening in the Southern Hemisphere subtropics and deep tropics.

This is the first study that directly attributes changes in precipitation trends to the influence of global warming.

The team compared observed changes in land precipitation during the twentieth century averaged over 10-degree bands of latitude with changes simulated by 14 climate models, divided into three groups. One group contained estimates of human greenhouse-gas emissions, one included only natural factors such as volcanic aerosols, and a third contained both.

The results show that climate change has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. The models including both human and natural influences gave the best fit to the observed trends.

In the zone between 40 and 70 °N, which includes much of North America and most of Europe, rainfall increased by 62 millimeters per century between 1925 and 1999. The researchers estimate that between 50 and 85% of this increase can be attributed to human activity.

The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel.

Because the observed changes were larger than the models predicted, current projections of future human impact on precipitation might be underestimates.

Although other studies have found human influence on climate in surface air temperature, sea level pressure, free atmospheric temperature, tropopause height, and ocean heat content, changes in precipitation had not previously been detected at the global scale. This is partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal, according to the authors.

The team used multiple data-sets of global rainfall pattern in the analysis. By examining individual latitudinal regions, the team was able to discover larger trends not accounted for by naturally occurring changes.